This guide explains how to work with the frame editor for editing source code, LaTeX documents, Command-line terminal etc. The screenshot below shows a Python file and a Terminal. In order to get that view, the frame editor is split vertically first, and then the right frame is splitted horizontally. After that, the bottom right frame is changed to a Terminal.

Therefore, you can not only view the same Python source code file in two places, but you can also work with e.g. IPython on the command-line in the same directory.

If you open/create a file ending with *.md, you’re presented by default with a split view of Markdown code on the left and HTML rendered output on the right hand side.
You can also write LaTeX formulas between $ signs, e.g. $\frac{1}{1+x^2}$.

A file ending in *.txt is a plaintext file, which does not have any formatting.
Similarly, you can also edit plaintext files with a special purpose, e.g. *.csv for CSV data,
*.yaml for structured data, and for source-code of software programs, there is
*.c, *.c++, *.sage, *.java, *.py, *.adb and many more!

Open/create a *.html file and you’ll see the rendered output on the right hand side. You can switch the rendered view between “Preview” (which is faster) or “iframe” (which renderes the page as it is) depending on your needs. You can even close the editor pane, to just see the rendered HTML.

Open a *.rmd file to work with RMarkdown.
Depending on the configuration in the preamble, you either produce an HTML or PDF output.
Do not forget to switch the panel for the rendered output accordingly.
(In case you produce PDF and HTML output, you can even see both after splitting the panel.)